The research objectives of this project include a characterization of the anatomical substrate and effects of narcotically mediated, descending neural modulation of neurophysiological and behavioral responses to noxious stimuli. These studies also allow inferences about precisely what activity of trigemino-thalamic neurons is necessary and/or sufficient for the perception of painful stimuli. Experiments include: (1) evaluation of the efficacy of morphine-produced analgesia in rats before and after lesions of specific spinal cord tracts including descending components of the dorsolateral funiculus; (2) behavioral assessment in Rhesus monkeys of the analgesic consequences, as measured by inhibition of flexion reflexes, of electrical stimulation of or narcotic microinjection into midbrain structures; (3) subsequent studies in the same animal of the effects of electrical stimulation or narcotic microinjection on responses of neurons in trigeminal nucleus caudalis responding to noxious and innocuous somatosensory stimuli; and (4) the gross evaluation of the narcotic specificity of all manipulations by administration of narcotic antagonists.